Electrically powered vehicles such as electric vehicles and hybrid vehicles have attracted a lot of attention as environmentally friendly vehicles. These vehicles incorporate a motor for generating a driving force for running, and a rechargeable power storage device for storing power supplied to the motor. A hybrid vehicle refers to a vehicle incorporating a motor as well as an internal combustion engine as a power source, a vehicle incorporating a power storage device as well as a fuel cell as a direct-current power supply for driving the vehicle, and the like.
As with an electric vehicle, a hybrid vehicle having a vehicle-mounted power storage device that can be charged from a power supply outside of the vehicle is known. For example, a so-called “plug-in hybrid vehicle” is known in which a power storage device can be charged from a power supply at an ordinary household by connecting a power supply outlet provided at the house to a charging inlet provided on the vehicle by a charging cable.
Wireless power transmission without using a power cord or a power transmission cable has been receiving attention in recent years as a power transmission method. Power transmission using electromagnetic induction, power transmission using a microwave, and power transmission by resonance are three dominant techniques known for wireless power transmission.
Among these techniques, the resonance is a contactless power transmission technique for causing a pair of resonators (e.g., a pair of coils) to resonate with each other in an electromagnetic field (near field) to transmit power through the electromagnetic field, and can transmit a large amount of power of several kW across a relatively long distance (e.g., several meters).
Japanese Patent Laying-Open No. 2010-141976 (Patent Document 1) discloses a contactless power transmitting apparatus for transmitting power in a contactless manner to a vehicle by resonance. This contactless power transmitting apparatus includes an alternating-current power supply, a primary coil connected to the alternating-current power supply, a primary-side resonant coil, a secondary-side resonant coil, and a secondary coil connected to a load (secondary battery), and further includes a variable impedance circuit arranged between the alternating-current power supply and the primary coil. The primary coil, primary-side resonant coil, secondary-side resonant coil, secondary coil and load constitute a resonant system. An impedance of the variable impedance circuit is adjusted to match an input impedance of the resonant system at a resonant frequency to an impedance of a portion closer to the alternating-current power supply than the primary coil.
A contactless power transmitting apparatus described in Japanese Patent Laying-Open No. 2010-141977 (Patent Document 2) includes an alternating-current power supply, a primary coil connected to the alternating-current power supply, a primary-side resonant coil, a secondary-side resonant coil, and a secondary coil connected to a load (secondary battery), and further includes a variable impedance circuit arranged between the secondary coil and the load. The primary coil, primary-side resonant coil, secondary-side resonant coil, secondary coil, load and variable impedance circuit constitute a resonant system. The alternating-current power supply outputs an alternating voltage at a frequency equal to a resonant frequency of the resonant system. An impedance of the variable impedance circuit is adjusted to suppress variation in input impedance of the resonant system.
According to these contactless power transmitting apparatuses, electric power can be efficiently supplied from the alternating-current power supply to the load without changing the frequency of the alternating-current power supply, even if the distance between the resonant coils or the load receiving the power is varied (see Patent Documents 1 and 2).